In order to approximate the appearance of continuous-tone (photographic) images via ink-on-paper printing, the commercial printing industry relies on a process known as halftone printing. In halftone printing, color density gradations are produced by printing patterns of dots or areas of varying sizes, but of the same color density, instead of varying the color density continuously as is done in photographic printing.
There is an important commercial need to obtain a color proof image before a printing press run is made. It is desired that the color proof will accurately represent at least the details and color tone scale of the prints obtained on the printing press. In many cases, it is also desirable that the color proof accurately represent the image quality and halftone pattern of the prints obtained on the printing press. In the sequence of operations necessary to produce an ink-printed, full-color picture, a proof is also required to check the accuracy of the color separation data from which the final three or more printing plates or cylinders are made. Traditionally, such color separation proofs have involved silver halide photographic, high-contrast lithographic systems or non-silver halide light-sensitive systems which require many exposure and processing steps before a final, full-color picture is assembled.
Colorants that are used in the printing industry are insoluble pigments. By virtue of their pigment character, the spectrophotometric curves of the printing inks are often unusually sharp on either the bathochromic or hypsochromic side. This can cause problems in color proofing systems in which colorants, as opposed to pigments, are being used. It is very difficult to match the hue of a given ink using a single colorant.
In U.S. Pat. No. 5,126,760, a process is described for producing a direct digital, halftone color proof of an original image on a colorant-receiving element. The proof can then be used to represent a printed color image obtained from a printing press. The process described therein comprises:
a) generating a set of electrical signals which is representative of the shape and color scale of an original image; PA1 b) contacting a colorant-donor element comprising a support having thereon a colorant layer and an infrared-absorbing material with a first colorant-receiving element comprising a support having thereon a polymeric, colorant image-receiving layer; PA1 c) using the signals to imagewise-heat by means of a diode laser the colorant-donor element, thereby transferring a colorant image to the first colorant-receiving element; and PA1 d) retransferring the colorant image to a second colorant image-receiving element which has the same substrate as the printed color image. PA1 wherein: PA1 a) a colorant-donor element as described above, and PA1 b) a colorant-receiving element as described above,
In the above process, multiple colorant-donors arc used to obtain a complete range of colors in the proof. For example, for a full-color proof, four colors: cyan, magenta, yellow and black are normally used.
By using the above process, the image colorant is transferred by heating the colorant-donor containing the infrared-absorbing material with the diode laser to volatilize the colorant, the diode laser beam being modulated by the set of signals which is representative of the shape and color of the original image, so that the colorant is heated to cause volatilization only in those areas in which its presence is required on the colorant-receiving layer to reconstruct the original image.
In color proofing in the printing industry, it is important to be able to match the proofing ink references provided by the International Prepress Proofing Association. These ink references are density patches made with standard 4-color process inks and are known as SWOP.RTM. (Specifications Web Offset Publications) Color Aims. For additional information on color measurement of inks for web offset proofing, see "Advances in Printing Science and Technology", Proceedings of the 19th International Conference of Printing Research Institutes, Eisenstadt, Austria, June 1987, J. T . Ling and R. Warner, p.55.
Infrared absorbing colorants are used in colorant-donor elements for laser-colorant transfer for the purpose of absorbing the laser energy and converting the radiant energy into thermal energy in order to cause colorant transfer to a receiver element. One problem encountered in the use of infrared colorants is that these colorants often exhibit some absorption in the visible spectrum. In the event that some or all of the infrared colorant is transferred along with the colorant, this absorption may spoil the color purity or hue of the transferred image colorant.
U.S. Pat. No. 5,972,838 discloses infrared absorbing dyes for laser colorant transfer. However, there is a problem in using these dyes under conditions of high humidity. When the dye-donor element is placed in a chamber at 38.degree. C./90% RH and kept under these conditions for 48 hours, the absorption at 810 nm decreases by 30-40%. This has been caused by a shift in the absorption of the IR dye to shorter wavelength probably due to aggregation. The dye is not being destroyed as it can be shown by HPLC analysis that most of the dye is still present.
It is an object of this invention to provide a colorant-donor element for laser-induced thermal colorant transfer containing a stabilizer which is useful under high humidity keeping conditions in maintaining maximum absorptivity of the infrared-absorbing cyanine colorant at the wavelength of the laser emission.